Anomalous precursor diamagnetism at low reduced magnetic fields and the role of T_C inhomogeneities in the superconductors Pb₅₅In₄₅ and underdoped La_1.9Sr_0.1CuO₄

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Lucía Cabo, Félix Soto, Mauricio Ruibal, Jesús Mosqueira, and Félix Vidal
LBTS, Departamento de Física da Materia Condensada, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain

Received 25 October 2005; revised 10 February 2006; published 18 May 2006

Themagnetic field dependence of the magnetization was measured above thesuperconducting transition in a high-T_C underdoped cuprate (La_1.9Sr_0.1CuO₄) and ina low-T_C alloy (Pb₅₅In₄₅). Near the superconducting transition [typically for(T–T_C)/T_C5×10^–2] and under low applied magnetic field amplitudes [typically forH/H_C2(0)10^–2, where H_C2(0) is the corresponding upper critical field extrapolatedto T=0 K] the magnetization of both samples presents a diamagneticcontribution much larger than the one predicted by the GaussianGinzburg-Landau (GGL) approach for superconducting fluctuations. These anomalies have beenalready observed in cuprate compounds by various groups and attributedto intrinsic effects associated with the nature of these high-T_Csuperconductors itself. However, we will see here that our resultsin both high- and low-T_C superconductors may be explained quantitatively,and consistently with the GGL behavior observed at higher fields,by just taking into account the presence in the samplesof a uniform distribution of T_C inhomogeneities. These T_C inhomogeneities,which may be in turn associated with chemical inhomogeneities, wereestimated from independent measurements of the temperature dependence of thefield-cooled magnetic susceptibility under low applied magnetic fields. These conclusionsare further confirmed by some additional low field magnetization measurementsin a Pb₉₂In₈ alloy and in pure Pb. The resultssummarized here also fully confirm the intrinsic character of ourprevious measurements of the fluctuation induced diamagnetism in La_1.9Sr_0.1CuO₄ undermoderate magnetic field amplitudes [up to H/H_C2(0)~0.2] as well asthe corresponding suggestions that the precursor Cooper pairs are notaffected by the presence of a pseudogap in the normalstate in this underdoped cuprate.

URL:	http://link.aps.org/doi/10.1103/PhysRevB.73.184520
DOI:	10.1103/PhysRevB.73.184520
PACS:	74.40.+k; 74.25.Ha; 74.72.Dn; 74.81.-g 74.40.+k Fluctuations in superconductors including noise, chaos, nonequilibrium superconductivity, localization, etc 74.25.Ha Magnetic properties of superconductors 74.72.Dn La-based cuprates (HTSC) 74.81.-g Inhomogeneous superconductors and superconducting systems YEAR: 2006
KEYWORDS:	diamagnetic materials, superconducting transition temperature, lead alloys, indium alloys, lanthanum compounds, strontium compounds, magnetisation, high-temperature superconductors, superconducting critical field, Ginzburg-Landau theory, magnetic susceptibility, fluctuations in superconductors

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